Gas chromatography (GC) is used to analyze and detect the presence of many different substances in a gaseous or vaporized sample. Gas chromatography uses various types of detectors, depending on the specific element or compound sought to be detected. Different detectors are used to achieve selective and/or highly sensitive detection of specific elements or compounds in particular chromatographic analyses.
Typically, a flame photometric detector (FPD) is used to detect the presence of sulfur or phosphorus in a particular sample, or analyte. A flame photometric detector uses what is referred to as a chemiluminescent reaction where compounds containing sulfur or phosphorus encounter a hydrogen-rich flame. Chemiluminescence uses quantitative measurements of the optical emission from excited chemical species to determine analyte concentration. Chemiluminescence is typically emission from energized molecule species. When burned, or combusted, in such a flame, sulfur is transformed into an emitting species referred to as S2* and phosphorus is transformed into an emitting species referred to as HPO*. The emission wavelength range for excited S2 includes, among others, the region from 320-405 nanometers (nm) and the wavelength range for excited HPO includes, among others, the range from 510-530 nm. The molecular emissions impinge on a photomultiplier tube, which converts photons to an electrical signal to quantify the concentration of a particular excited species.
FPDs often incorporate a photomultiplier tube (PMT) to measure the number of photons and thus the intensity of light emitted from phosphorus and sulfur containing compounds, with wavelength selective filters disposed between the flame of the FPD and the PMT.
Generally, it is beneficial for FPD's to operate at temperatures equal to or exceeding the highest oven temperature reached during the analysis so that compounds eluting from the GC column do not condense before reaching the flame of the FPD. Such condensation would result in the measurement of inaccurate intensities of specific compound species, and ultimately in inaccurate measurements. Furthermore, there is a response temperature dependence that impacts parameters such as sensitivity and baseline noise of the FPD.
While it is desirable to operate the FPD at temperatures equal to or greater than the highest oven temperature reached during analysis, the PMT must be maintained at a comparatively low temperature to prevent the background noise of the PMT from impacting the accuracy of measurements. So, it is desirable to maintain the transfer line to the emission block of the FPD at a comparatively high temperature (e.g., 250° C.), while maintaining the PMT at a comparatively low temperature (e.g., 50° C.)
Additionally, there is demand to maintain the transfer line section of the FPD at even higher temperatures (e.g., 400° C.). Unfortunately, many current FPDs are limited to operation at approximately 250° C., above which mechanical failures can result. For example, seals between the transfer line and the detector block, between the flame igniter and the detector block, and around the window of the PMT can fail. The failure of these seals can initially impact the baseline noise of the PMT, thereby affecting accuracy of measurements.
What is needed, therefore, is an apparatus that overcomes at least the shortcomings of known structures described above.